At first, some definitions and nomenclatures which will be used throughout the text are provided below for a better understanding of the text.
FDS [Hz]: Switching frequency of the suction lines, that is, the frequency with which the flow of the refrigerant gas is switched between the two suction lines and, consequently, between the two refrigeration circuits.
PDS [s]: Switching period of the suction lines, that is, period of time in a switching cycle of both suction lines is completed. Inverse of FDS.
DDS [%]: Suction duty cycle, that is, when there are two suction lines, where the flow of the refrigerant gas through the second line complements that of the first line, there will be a duty between the conduction time of each line and the period PDS. It is a duty cycle once it refers to the times existing in a switching period of the suction lines, being possible to vary it in every new period. To identify the duty cycle of each suction line, D1DS is established as the duty cycle of the first suction line and D2DS is established as the duty cycle of the second line. The sum of D1DS and D2DS must be equal to one, therefore DDS refers to the set of values (D1DS, D2DS), for instance, (80, 20%), (20, 80%), (50, 50%), etc.
RPMDS: Rotation of the internal motor of the double suction compressor. It can be a fixed value or zero for conventional fixed capacity compressors (or compressor ON-OFF) or any value within a range of operation, for variable capacity compressors. In a double suction compressor, the value of RPM can be defined for each suction line, as RPMEV1 and RPMEV2. The refrigeration capacity of a compressor is proportional to the rotation of the internal motor of the compressor or proportional to the other form of pumping the refrigeration gas, for instance, by means of linear actuators.
CAPCOMP: Refrigeration capacity of a compressor, wherein the capacity value can be a single one or specific for each suction line (CAPCOMP1 and CAPCOMP2).
TDS [N·m]: Double suction compressor's motor load; that is variable or fixed speed motor. The load will be specific for each one of the two suction lines (T1DS and T2DS). The load processed by the motor can be obtained directly or indirectly through the acquisition of electrical signals from the motor (voltage, current, phase differences, etc).
Nomenclature adopted in the sequence for elements employed in refrigeration systems:
CDS (Double Suction Control) Device for activating a valve in a double suction compressor—Electronic circuit capable of activating the double suction compressor's internal valve, in a duty cycle DDS.
SET (Temperature State Sensor)—Any contact or electrical signal whose state is changed, between two levels, according to certain temperature values, forming a hysteresis window. For instance; electromechanical thermostat and electronic thermostat with relay output to activate a compressor, or an electronic thermostat with digital output to control another actuator which activates the compressor.
SCT (Continuous Temperature Sensor)—Any sensor which delivers a physical quantity (generally voltage or electric current) proportional to a temperature value (NTC, PTC, etc.).
STQ (Load Sensor)—Electronic circuit which provides an electrical signal proportional to the load being processed by the compressor's motor.
ETH (Electronic Thermostat)—Electronic circuit whose main role is to interpret the states or values of the SETs and SCTs and to activate or send a drive control to the compressor.
TSD (Time Starting Device)—Electronic circuit responsible for performing the controlled start-up of a single-phase induction motor employed in fixed capacity compressors.
I-VCC (Inverter of Variable Capacity Compressor)—Electronic circuit called Frequency Inverter, responsible for activating the motor or actuator present in variable capacity compressors.
CVC (Capillary Tube Valve Control) Device for driving the valve that regulates the restriction of the capillary element—Electronic circuit capable of activating a valve positioned in series with the capillary tube of the refrigeration circuit, at a certain frequency and duty cycle.
Double Suction Compressor
The double suction compressor consists of a compressor having two suction lines whose switching occurs internally to the compressor, at a complementary work cycle. Switching occurs by means of a valve, which, on switching once in every period of time PDS, distributes the gas flow measurement through one of the suction lines in a period D1DS×PDS, and through the second suction line in a period (1−D1DS)×PDS. Valve switching is performed through an electric current applied by an external actuator CDS.
The Possible Configurations of the Refrigeration System
The double suction compressor, having a variable or fixed speed actuator or motor, can be employed in different types of refrigeration systems, classified according to their complexity. This classification is made to make it easier to understand the control methods to be proposed, once they are suitable for different goals of cost, efficiency, performance, etc.:
Low Complexity System:
It prioritizes a competitive product through the lowest cost/price of the elements employed. In general, it uses a compressor with fixed rotation motor (“ON-OFF compressor”), electromechanical thermostat with temperature hysteresis control (on, off). In some cases, the thermostat can be electronic to obtain better adjustment of the hysteresis window of controlled temperatures.
Medium Complexity System:
It prioritizes a competitive product through the balance between cost and performance by consumption or temperature control. In general, an additional element, or an element of higher complexity, is used to improve temperature control in one or more compartments, or to reduce energy consumption. For instance, this element can be a compressor with variable displacement or speed actuator or motor (Variable Capacity Compressor, or “VCC compressor”, also designated as having capacity performed through the phased variation in its operation state), or flow measurement valves at the capillary elements of each refrigeration circuit. The thermostat can be both electromechanical and electronic.
High Complexity System:
It prioritizes a competitive product through better performance (lower consumption, better temperature control, better design, etc.). In general, a configuration having several elements of higher complexity is used. For instance, this configuration can have a variable capacity compressor, flow measurement valves at the capillary elements, electronic thermostat that reads several sensors distributed in each compartment, etc.